Cement structures such as concrete and mortar structures in sewage treatment facilities and the like conventionally have problems of becoming gypseous and deteriorated. This deterioration is ascribable to sulfur-oxidizing bacteria of the genus Thiobacillus that oxidize hydrogen sulfide to produce sulfuric acid. These bacteria usually live widely in soil or water, and grow by oxidation of sulfur compounds and assimilation of carbon dioxide. Some species of the sulfur-oxidizing bacteria of the genus Thiobacillus live in structures in contact with sewage water, yet some species of the bacteria live in structures exposed to the air and not in contact with sewage water in the sewage treatment facilities.
For preventing deterioration of such structures, various methods have hitherto been proposed. At present, practically employed methods include coating the structures with corrosion resisting materials. However, the coatings of the corrosion resisting materials are easily damaged from pinholes or scratches, and thus have problems in durability. In addition, it is difficult to coat the parts having complex configurations and fine parts such as joints of small-diameter tubes.
It is known that heavy metal ions could kill the sulfur-oxidizing bacteria of the genus Thiobacillus. Such bactericidal action is exhibited by direct action of heavy metal ions on the bacteria. Since heavy metals are dissolved and released in an ionized state, a large amount of heavy metals are required for achieving the bactericidal action. Released ions of some kinds of heavy metals have seriously harmful effect on environment, so that such metals cannot be used in sewage treatment facilities.
To overcome these problems, there is proposed and practiced a method for preventing deterioration of cement structures induced by sulfur-oxidizing bacteria of the genus Thiobacillus, wherein particular metals or metal oxides that are insoluble in water but soluble in sulfuric acid, such as nickel, are added to structures such as those of concrete (JP-A-4-149053). This method is excellent in that the particular metals completely prevent the sulfur-oxidizing activity, respiration, and carbon dioxide-fixation activity of the sulfur-oxidizing bacteria in the neutral pH range, resulting in sufficient prevention of deterioration of cement structures induced by sulfur-oxidizing bacteria.
However, inhibitory activities of nickel or its oxides against the bacteria of the genus Thiobacillus decrease toward the acidic pH range, and hardly exhibited at pH 3 or below. Therefore, the inhibitory activity of such metals can be maintained sufficiently in the area usually in contact with sewage water where the pH value is kept near neutrality, but hardly exhibited in acidic area not in direct contact with sewage water, or when sewage water is transiently acidified. In addition, when the metals that are insoluble in water but soluble in sulfuric acid, such as nickel, are exposed to strong acid, such metals are dissolved and released as metal ions, thus being unpreferable in the environmental point of view. The release of the metal ions also reduces the volume of the inhibitor itself, causing difficulty in maintaining the inhibitory activity against the bacteria of the genus Thiobacillus for a prolonged period of time.
Metals having inhibitory activities against the bacteria of the genus Thiobacillus other than nickel have also been under research. For example, it is known that a mixture of molybdenum, ammonium molybdate, or ammonium molybdate and tungsten activates growth of Thiobacillus novellus, whereas tungsten, when used alone, inhibits growth of the same bacteria (Journal of Bacteriology, Vol. 153, No. 2 (1983) William M. et al. "Sulfite Oxidase Activity in Thiobacillus novellus" p.941-944). It is also reported that molybdenum (Mo.sup.4+), which is known to activate growth of the above-mentioned Thiobacillus novellus, inhibits growth of Thiobacillus thiooxidans (Chemical Abstracts, Vol. 95, No. 1 (Jul. 6, 1981) p127 (1081a)).
Thus, even the sulfur-oxidizing bacteria of the same genus of Thiobacillus have different growth inhibitory mechanism. Therefore, even if a certain growth inhibitor is demonstrated to have an effect on Thiobacillus novellus viable in the neutral pH range of 6 to 8, such an inhibitor will not be used in the acidic pH range of 2 to 6, where Thiobacillus novellus is hardly viable. Further, it is not believed that a substance that inhibits growth of Thiobacillus novellus equally inhibits growth of Thiobacillus thiooxidans viable in the pH range of 2 to 6. Recent researches therefore indicate that the growth inhibitors proposed hitherto are not effective to all species of the sulfur-oxidizing bacteria of the genus Thiobacillus.